1. Field of the Invention
The present invention relates to a video delivering system that delivers video streams, and more particularly, to a technology to control the intervals of delivery of a plurality of video streams.
2. Description of the Related Art
The recent diffusion of network technology has led to the development of technology to send video streams by means of Internet Protocol (IP). The technology of sending video streams falls under two categories: one that delivers video streams in real time by means of IP multicasting, and one that downloads video streams and then plays back accumulated video data. Among the known systems for encoding video data is MPEG2 (Moving Picture Experts Group 2). Among the known systems for delivering video streams is RTP/UDP (Real-time Transport Protocol/User Datagram Protocol).
The increased bandwidth of communication lines permits delivery of plural video images. For example, live telecasting of sports (baseball) employs plural cameras, but the viewer cannot watch specific images he wants because only images selected by the broadcasting station from images of several cameras are delivered. However, if it is possible to deliver plural images at the same time (and hence to deliver images from all cameras), then the viewer will be able to watch several images at the same time or to select arbitrary images he wants. In the case of live telecasting of baseball, for example, the viewer will be able to watch the manager and other players while the pitcher is throwing a ball. In other words, the viewer will be able to watch several images of objects at the same time which cannot be taken by a single camera.
What is important in simultaneous delivery of plural images is synchronization of images. In the case of a transport stream of MPEG2, the reference time information to be used for decoding and display of video frames is generated for individual streams according to PCR (Program Clock Reference) contained in TS packets. Therefore, the delivery of plural streams through broad IP networks is subject to jitter caused by the network. Thus the timing (generated by clock) of each stream does not coincide with the rate of clock advancing, which results in discrepancy between the indicated timing and the start of image playback for each stream.
Also, in the case of an IP network, which does not guarantee the arrival of packets, there is a possibility that IP packets containing information required for clock generation are discarded in the network.
One method for synchronous playback of plural images is disclosed in Japanese Patent Laid-open Publication No. 2001-19704. This synchronizing system selects a stream (that functions as a master) from plural streams and decodes other streams by using the clock of the selected stream.